1. Field of the Invention
The present invention relates to a noise cancellation circuit of a touch panel, and particularly to a noise cancellation circuit of a touch panel that can utilize a noise antenna installed in the touch panel or a noise antenna installed in a liquid crystal panel to cancel noise of the touch panel and noise of the liquid crystal panel received by a readout circuit.
2. Description of the Prior Art
General speaking, most multi-touch panels are mutual capacitance touch panels. Please refer to FIG. 1. FIG. 1 is a diagram illustrating a driving method of a touch panel 100 detecting at least one touch point. As shown in FIG. 1, the touch panel 100 can detect positions of a plurality of touch points on the touch panel 100 accurately. The driving method of the touch panel 100 utilizes voltage scanning signals or current scanning signals to scan sensing lines in turn in a Y direction (or an X direction) of the touch panel 100, and then to capture feedback signals from all sensing lines in the X direction (or the Y direction) of the touch panel 100.
Please refer to FIG. 2. FIG. 2 is a diagram illustrating a readout circuit 102 of the touch panel 100 according to the prior art. As shown in FIG. 2, the touch panel 100 generates a signal X or a signal Y according to a position of an object 104 on the touch panel 100, and transmits the signal X or the signal Y to a multiplexer 108 through a flexible printed circuit (FPC) 106. Then, the multiplexer 108 transmits the signal X or the signal Y to the readout circuit 102, and the readout circuit 102 executes a signal process (such as amplifying and filtering) on the signal X or the signal Y to generate a signal X′ or a signal Y′. Finally, the signal X′ or the signal Y′ is transmitted to a next stage circuit 112 (such as an analog-to-digital converter or a microprocessor) to be processed by executing a corresponding operation.
Please refer to FIG. 3. FIG. 3 is a diagram illustrating operation of the touch panel 100. As shown in FIG. 3, when a switch SW1 is turned on and a switch SW2 is turned off, a signal source S (a signal Y) generated by the touch panel 100 utilizing a voltage scanning signal or a current scanning signal to scan a first sensing line in the Y direction charges a capacitor Cx to make the capacitor Cx store a voltage V. When the switch SW1 is turned off and the switch SW2 is turned on, the multiplexer 108 transmits the voltage V stored in the capacitor Cx to the readout circuit 102 in the X direction. Then, the readout circuit 102 executes a signal process on the voltage V. On the other hand, when the switch SW1 is turned on and the switch SW2 is turned off, if a signal source S is generated by the touch panel 100 utilizing a voltage scanning signal or a current scanning signal to scan one sensing line in the X direction, the multiplexer 108 transmits a voltage V stored in a capacitor Cy to the readout circuit 102 in the Y direction when the switch SW1 is turned off and the switch SW2 is turned on. Then, the readout circuit 102 executes a signal process on the voltage V stored in the capacitor Cy. Then, a signal V′ generated by the readout circuit 102 is transmitted to the next stage circuit 112 to be processed by executing a corresponding operation.
Please refer to FIG. 4 and FIG. 5. FIG. 4 is a diagram illustrating noise sources in FIG. 1, and FIG. 5 is a diagram illustrating noise included by the signal V′ generated by the readout circuit 102 when the touch panel 100 operates. As shown in FIG. 1 and FIG. 4, the noise in FIG. 1 includes noise N1 generated by a liquid crystal panel 114 coupled to the touch panel 100, noise N2 generated by the touch panel 100 due to external interference, noise N3 generated by the multiplexer 108, and noise N4 generated by the readout circuit 102. Therefore, as shown in FIG. 3 and FIG. 5, the readout circuit 102 generates the signal V′ according to the voltage V stored in the capacitor Cx or the capacitor Cy, the noise N1, the noise N2, the noise N3, the noise N4, and equation (1). In addition, a signal-to-noise ratio SNR can be determined according to equation (2):V′=V+N1+N2+N3+N4  (1)SNR=V/(N1+N2+N3+N4)  (2)
Thus, the signal V′ generated by the readout circuit 102 according to the signal V may make the next stage circuit 112 execute an error operation.